Carbon nanotubes are a class of nanomaterials that have the potential to provide a variety of new, and previously unattainable, combinations of properties to materials. One emerging area is solution-processed, flexible, conductive coatings. Carbon nanotubes can be dispersed in a medium, and processed by conventional solution processing methods to give flexible, conductive layers, and, if desired, transparent, flexible conductive layers. Another area is conductive polymer composites. Carbon nanotubes can be dispersed in a polymer matrix to give a conductive polymer composite that retains the mechanical and processing properties of the polymer.
A key issue for most applications employing carbon nanotubes is dispersion. The carbon nanotubes must be dispersed in a medium that is compatible with the process and/or with the polymers. Owing to their high surface area and small diameters, carbon nanotubes are very difficult to disperse and many of the typical methods used for dispersing conventional pigments are not effective.
The ideal dispersion method would employ a combination of dispersing agents and solvents that are amenable to a variety of processes. For example, spraying requires the use of volatile solvents, coating on polymer substrates requires the use of solvents that do not etch the substrate, and blending with polymers requires systems that are compatible with the monomer or polymer. The dispersing agents and solvents should have minimal environmental impact and health risks. Finally, the method should not negatively affect the performance properties, such as the conductivity, of the nanotubes. Ideally, the dispersant should be “fugitive” once it has performed its intended role of dispersing CNTs and thus aid in realizing the desired electrical and other properties of CNTs.
The most common method described in the open literature is the use of aqueous surfactant solutions, such as sodium dodecylbenzene sulfonate, sodium lauryl sulfate, and Triton X-100. High quality dispersions can be prepared by suspending the carbon nanotubes in the surfactant solution at dilute concentration, sonicating, and centrifuging. Such aqueous dispersions are suitable for only a limited number of applications. Most polymers and their monomers are not soluble in water; therefore, aqueous dispersions are not suitable for many multifunctional coatings and nanocomposite applications. In these dispersions, the weight ratio of surfactant to nanotube is quite high. Therefore, these dispersions are not appropriate for preparing conductive coatings by most solution processing methods unless the surfactant is removed during or after film formation.
Another common method described in the literature is the use of dichlorobenzene. Carbon nanotubes can be dispersed in dichlorobenzene after ultrasonication. Dichlorobenzene overcomes some of the problems of aqueous solutions, since it is compatible with more organic monomers and polymers and leaves little additional residue upon drying. However, it is high boiling, hazardous for health and environment, and etches many polymer substrates.
Finally, non-polar, aprotic solvents such as N-methylpyrollidione (NMP), dimethylformamide (DMF), and dimethylacetamide (DMAC) are often used. Similarly, aliphatic amines have been used to disperse SWNTs in solvents such as THF. Unfortunately, these systems tend to decrease the conductivity, particularly if no high temperature drying step is used to remove them, as in the case of polymer composite formation.